Modify static type of operand.
The following construct(s) refer to this construct:
A TreatExpr modifies the static type of its operand and
takes an expression and a sequence type as arguments. It does not change the
dynamic type; rather, it ensures that an expression has an expected dynamic
type at evaluation time.
The static type of $myaddress may be
Address, a less specific type than USAddress.
However, at run-time, the value of $myaddress must
match the type USAddress using rules for SequenceType matching;
otherwise a dynamic error is raised.
$myaddress treat as element()
